A previously undefined autosomal dominant muscle disease has been identified in a five generation family (MN1). Although this family has the clinical features associated with myotonic dystrophy (DM), affected individuals do not have the CTG expansion associated with DM, nor is the disease locus linked to the DM region of chromosome 19. Linkage to other genetic loci that cause myotonia have been excluded. We propose to genetically map the MN1 locus, first by performing a genome screen and then by developing a high resolution genetic map for the MN1 region. The identification of a second locus for DM will facilitate the identification of additional families with the MN1 form of DM and will be a first step toward identifying the MN1 gene. Because the MN1 disorder causes the same broad range of clinical features as classic DM, the eventual isolation and characterization of the gene(s) affected will provide insights into the pathology of both DM and the disease afflicting the MN1 family.